1. Field of the Invention
This invention relates to an electrophotographic type of image forming apparatus which uses a contact-and-charge method of bringing a charging member into contact with a rotating photosensitive body in order to charge the photosensitive body.
2. Description of the Prior Art
An electrophotographic type of image forming apparatus, such as an electrostatic copying machine, a photoprinter, a facsimile, or the like, includes a unit for charging a photosensitive body to be charged and a unit for transferring a toner image formed on the surface of the photosensitive body to paper in a transferring process. In this type of image forming apparatus, conventionally, a noncontact-and-charge method, i.e., corona discharge method has been widely used. According to the corona discharge method, a high voltage is applied to tungsten wires called corona wires so as to carry out a discharge. For this reason, air is ionized and, as a result, large quantities of ozone and nitrogen oxide are generated. These are injurious to persons and exert a harmful influence on the environment. The ozone and nitrogen oxide adhere to the photosensitive body, charging member, optical systems, etc. of the image forming apparatus, thereby causing the deterioration of images.
Especially in the case of a minus discharge, ozone is conspicuously generated. In recent years, the generation of ozone has assumed a serious phase because an organic photosensitive body for the minus discharge has been widely used as a photosensitive body and because an environmental standard for generated gas has become rigid.
Accordingly, as a charging unit for resolving the above disadvantages, a contact-and-charge type of charging unit is being watched with keen interest. In the contact-and-charge type of charging unit, e photosensitive body is charged by contact with a charging roller as a charging member to which a voltage is applied.
An example of such a contact-and-charge type of charging unit is known as disclosed in, for example, Japanese Patent Application Early Laid-Open Publication Hei 3-48870. In the charging unit disclosed therein, a charging roller as a charging member is designed to be brought into and out of contact with a photosensitive body as an image carrier by means of a solenoid. The charging roller is brought into contact with the photosensitive body when the photosensitive body is charged by the charging roller whereas the charging roller is kept away from the photosensitive body when the photosensitive body is not charged by the charging roller. In this connection, there is also known a charging unit in which, when a photosensitive body is not charged by a charging roller, the charging roller is brought into contact with the photosensitive body by pressure smaller than that required when the photosensitive body is charged by the charging roller.
As shown in FIG. 18, a charging unit is also known in which a charging roller 62 is always pressed against a photosensitive body 60 by means of a spring 61 so that the charging roller 62 is brought into contact with the photosensitive body 60 and is rotated following the rotation of the photosensitive body 60, and a cleaning member 63 is arranged to be brought into and out of contact with the charging roller 62 by operating a solenoid 65 with predetermined timing.
Further, as disclosed in Japanese Patent Application Early Laid-Open Publication Hei 3-130787, a charging unit is known in which a charging roller supported by a lever so as to come in contact with a photosensitive drum or separate therefrom is kept away from the photosensitive drum and, at the same time, a cleaning member, such as a cleaning roller, a cleaning pad, or the like, is brought into contact with the charging roller when the photosensitive drum is not charged and before a power switch (copy key switch) is turned on. When the power switch is turned on, the charging roller is rotated keeping in contact with the cleaning member, so that the surface of the charging roller is cleaned. In addition, this publication (Hei 3-130787) discloses that it is desirable that the charging roller is rotated away from the photosensitive drum in order to clean the whole surface of the charging roller.
Further, as disclosed in Japanese Patent Application Early Laid-Open Publication Hei 3-188738, a charging unit is known in which a charging roller supported by a lever so as to come in contact with a photosensitive drum or separate therefrom is kept away from the photosensitive drum when the photosensitive drum is not charged whereas the charging roller is brought into contact with the photosensitive drum when the photosensitive drum is charged, and the driving force of the photosensitive drum is transmitted to the charging roller via a belt when the charging roller is separated from the photosensitive drum.
Referring again to the charging unit shown in FIG. 18, the circumferential speed (surface peripheral speed) of the surface of the charging roller 62 is constant whether the cleaning member 63 is cleaning the charging roller 62 or not because the charging roller 62 is rotated always following the rotation of the photosensitive body 60. In a case where the charging roller 62 is thus rotated by directly obtaining the driving force from the photosensitive body 60, the circumferential speed of the surface of the charging roller 62 is influenced by a speed (i.e., a process linear velocity corresponding to the circumferential speed of the surface of the photosensitive body 60) at which an image is formed. Therefore, in an image forming apparatus high in process linear velocity, the circumferential speed of the surface of a charging roller with which a cleaning member is kept in contact is increased. Accordingly, owing to heat generated by the rubbing of the cleaning member against the charging roller, a toner is liable to adhere to the photosensitive body or to the charging roller, or the surface of the charging roller might easily receive mechanical damage.
On the other hand, in a charging unit disclosed in Japanese Patent Application Early Laid-Open Publication Hei 5-188738, a belt is loose immediately after a charging roller is separated from a photosensitive body. In other words, the belt is not so tense as to transmit the driving force to the charging roller. For this reason, the rotation of the charging roller is once stopped, and then the belt is tensioned when the charging roller is separated from the photosensitive body by a predetermined distance, so that the rotation of the photosensitive body is transmitted to the charging roller. Therefore, the rotation of the charging roller starts a little late with respect to the timing of the separation of the charging roller from the photosensitive body. In addition, according to this arrangement, the belt also becomes loose when the charging roller is moved to come in contact with the photosensitive body and, therefore, the rotation of the charging roller is stopped immediately after the charging roller is separated from a cleaning member. Accordingly, the charging roller which is not being rotated is brought into contact with the photosensitive body which is rotating. This causes great abrasion of the cleaning member and damage to the photosensitive body. In addition, the driving duty of the charging member is greater than that in the case of kinetic friction.